(1) Field of the Invention
The present invention relates to a control valve for a variable displacement compressor, and more particularly to a control valve for a variable displacement compressor, for controlling discharging amount of refrigerant in the compressor forming a component of a refrigeration cycle for an automotive air conditioner.
(2) Description of the Related Art
A compressor used in the refrigeration cycle of an automotive air conditioner, for compressing refrigerant, uses an engine as a drive source, and hence is incapable of performing rotational speed control. To eliminate the inconvenience, a variable displacement compressor capable of varying the displacement of refrigerant is employed so as to obtain an adequate cooling capacity without being constrained by the rotational speed of the engine.
In such a variable displacement compressor, a wobble plate fitted on a shaft driven by the engine for rotation has compression pistons connected thereto, and by varying the inclination angle of the wobble plate, the stroke of the pistons is varied to vary the discharge amount of refrigerant.
The inclination angle of the wobble plate is continuously changed by introducing part of compressed refrigerant into a hermetically closed crankcase to cause a change in the pressure of the introduced refrigerant, thereby changing the balance of pressures acting on the opposite sides of each piston.
A control valve is disposed between a discharge chamber and a crankcase of the compressor, or between the crankcase and a suction chamber of the compressor, for adjusting pressure in the crankcase by changing the flow rate of refrigerant introduced from the discharge chamber into the crankcase, or changing the flow rate of refrigerant delivered from the crankcase to the suction chamber. For example, in the former case, an orifice is disposed between the crankcase and the suction chamber, and a path is formed through which refrigerant is allowed to flow from the discharge chamber into the suction chamber. The control valve includes a valve element which is moved to and away from a valve hole forming a refrigerant passage communicating e.g. between the discharge chamber and the suction chamber for opening and closing the valve hole. By driving a solenoid so as to control the lift of the valve element from the valve hole, the flow rate of refrigerant is adjusted which flows from the discharge chamber side to the suction chamber side (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2003-328936 (FIG. 2, etc.)).
More specifically, this control valve has the valve element disposed on the downstream side of the valve hole, and a shaft for axially supporting the valve element on a side of the valve element opposite from the valve hole. The shaft is integrally formed with a plunger (movable core) of the solenoid and is in contact with an end face of the valve element. The control valve includes a spring urging the valve element in the valve-opening direction, a spring interposed between the plunger and a core (fixed core), for urging the plunger in the valve-opening direction, and a spring for urging the plunger in the valve-closing direction. As a result, when the solenoid is energized, the valve element is held at a position where the pressure of refrigerant, the resultant force of the springs, and a solenoid force are balanced, whereby the valve opening degree of the control valve is determined.
In the control valve configured as above, if an urging force in the valve-closing direction becomes short as the valve element moves from its valve-closing position to an end position of a pressure control area over which pressure control is actually performed, the valve element suddenly moves to a fully-open position, in spite of the fact that the valve portion should be held at a predetermined valve opening degree. More specifically, if the force applied to the valve element in the valve-closing direction temporarily decreases in spite of the fact that the valve opening degree increases when the valve element is in the pressure control area, the valve element suddenly moves to the fully-open position when the force generated by the pressure of refrigerant in the valve-opening direction has exceeded a force as a starting point of the decrease. On the other hand, when the pressure of refrigerant is reduced by the fully-open state of the valve, the valve element moves to a fully-closed position again. The above motions of the valve element raise the problem that valve is repeatedly opened and closed, whereby it is impossible to realize stable pressure control in the pressure control area.
To solve this problem, conventionally, when the valve element is in the pressure control area, the urging forces of the springs are increased as the valve opening degree increases, whereby the force generated by the springs and the solenoid in the valve-closing direction and the force generated by the pressure of refrigerant in the valve-opening direction are balanced.
However, when the force generated by the springs in the valve-closing direction is increased as described above, this results in a decrease in the maximum valve opening degree. Therefore, it is impossible to ensure a sufficient flow rate of refrigerant when the valve is fully open, resulting in the degraded responsiveness in causing the compressor to shift to operation with the minimum displacement.